Abstract: Objective : To investigate the expression of MHC class I chain-related gene A (MICA) in some tumor specimens and tumor cells and the clinical significance. Methods The expression of MICA was measured by RT-PCR and by immunohistochemical method (SABC technique). The cytotoxicity of human NK cells were detected by MTT method. Results : The expression of MICA mRNA and protein was found in most tumor cell lines and tumor tissues, but was not found in paraneoplastic specimens. NK cells exert higher cytotoxicity to tumor cells with MICA expression, while tumors without MICA expression do resist against NK lysis. Conclusion : The expression of MICA on tumor cell surface can enhance the cytotoxicity of NK cells through interaction of MICA and NKG2D. Shedding of sMICA from tumor cells may promote tumor immune evasion.